def search4vowels(word: str)->str:
    '''search4vowels ： return the found string'''    
    vowels = ['a', 'e', 'i', 'o', 'u']
    found = list()
    for letter in word:
        if letter not in found and letter in vowels:
            found.append(letter)
    return ''.join(found)


print(search4vowels('hello world'))
print(search4vowels('aaabbbbiioud'))
